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Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 103,765	$ 119,057
Gross Unrealized Gains	14	2
Gross Unrealized Losses	(1)	(6)
Estimated Fair Value	103,778	119,053
Money Market Funds | Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,475	29,090
Estimated Fair Value	23,475	29,090
Money Market Funds | Restricted cash
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50	50
Estimated Fair Value	50	50
U.S. Government Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65,645	74,144
Gross Unrealized Gains	9	1
Gross Unrealized Losses	(1)	(6)
Estimated Fair Value	65,653	74,139
Cash and Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	23,475	30,089
Short-Term Investments
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	80,253	88,914
Federal Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,595
Gross Unrealized Gains	5
Estimated Fair Value	$ 14,600
Federal Agency Securities | Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		999
Estimated Fair Value		999
Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		14,774
Gross Unrealized Gains		1
Estimated Fair Value		$ 14,775

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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